1. Field of the Invention
The present invention relates to a method of flame abrasion of a glass preform to remove unevenness on its surface and smoothen its surface so as to provide a glass preform which is suitable for the fabrication of an optical fiber.
2. Description of the Related Art
As a method for producing a high purity quartz preform for an optical fiber, a VAD (Vapor Phase Axial Deposition) method is known and widely used. FIG. 1 schematically illustrates the VAD method. In the VAD method, SiO.sub.2 glass soot is synthesized using an oxyhydrogen flame 8 and deposited on a peripheral surface or a lower end of a starting member 6, for example, a quartz rod to form a porous glass preform 7 in an axial direction of the starting member 6. Then, the porous glass preform is sintered and vitrified to prepare a transparent glass preform.
Since the transparent glass preform has unevenness of several micrometers on its surface, the peripheral surface of the preform is smoothened by a flame abrasion with an oxyhydrogen flame. FIG. 2 illustrates a conventional flame abrasion method, in which a glass preform 2 is connected to supporting rods 1 which are attached to respective chucks 4, and the surface of the rotating glass preform 2 is heated with oxyhydrogen flames 3. In the conventional flame abrasion method, the glass preform is supported horizontally as shown in FIG. 2.
Recently, in view of the reduction of production cost of optical fibers, a diameter and a length of the preform for the optical fibers have been increased. When the glass preform is horizontally supported in the flame abrasion step, as a weight of the preform increases, too much weight is loaded on the supporting rods so that the rods may be broken.
As the diameter of the glass rod increases, a center part of the preform cannot be sufficiently heated only by heat of the oxyhydrogen flame during the flame abrasion and a large temperature difference is created between the center part and the peripheral surface part of the preform, whereby a thermal strain is generated in the glass preform, so that the preform often cracks during cooling after the flame abrasion.